Various methods and systems for establishing communication connections between the subscribers of a data network are known from the prior art. Bus systems by which every subscriber can directly address every other subscriber of the data network are in widespread use. Furthermore switchable data networks are known in which what are referred to as point-to-point connections can be established, i.e. a subscriber can only reach all other subscribers of the switchable data network indirectly, by corresponding forwarding of the data to be transmitted by means of one or more coupling units.
Data networks allow communication between a number of subscribers by networking, that is connecting the individual subscribers to each other. Communication here means the transmission of data between the subscribers. The data to be transmitted is sent in this case as data telegrams, i.e. the data is packed into a number of packets and sent in this form over the data network to the corresponding recipient. The term data packet is thus used. The term transmission of data is used in this document fully synonymously with the above-mentioned transmission of data telegrams or data packets.
Networking itself for example is implemented in switchable high performance data networks, especially Ethernet, by connecting at least one coupling unit between two subscribers in each case, which is connected to both subscribers. Each coupling unit can be connected to more than two subscribers. Each subscriber is connected to at least one coupling unit but not directly to another subscriber. Subscribers are for example computers, Programmable Logic Controllers (PLC) or other machines which exchange electronic data with other machines and especially process it.
In distributed automation systems, for example in the area of drive technology, specific data must arrive at specific times at the intended subscribers and must be processed by the recipients. This is referred to as realtime-critical data or realtime-critical data traffic since, if the data does not arrive at its intended destination at the right time this can produce undesired results at the subscriber. In accordance with the IEC 61491, EN61491 SERCOS interface—Brief Technical Description in German (http://www.sercos.de/deutsch/index deutsch.htm) successful realtime critical data traffic of the type mentioned can be guaranteed in distributed automation systems.
Likewise the use of a synchronous, clocked communication system with equidistance characteristics is known per se in such an automation system. This is taken to mean a system consisting of at least two subscribers that are linked via a data network for the purposes of mutual exchange of data or mutual transmission of data.
In this case data is exchanged cyclically in equidistant communication cycles which are specified by the communication clock used by the system. Subscribers are for example central automation devices, programming, project planning or operating devices, peripheral devices such as input/output modules, drives, actors, sensors, Programmable Logic Controllers (PLC) or other control units, computers or machines which exchange electronic data with other machines and process data, especially from other machines. In this document control units are taken to mean closed-loop controllers or control units of all types. Typical examples of data networks used are bus systems such as Field Bus, Profibus, Ethernet, Industrial Ethernet, FireWire or also PC-internal bus systems (PCI), etc.
Automation components (e.g. controllers, drives, . . . ) nowadays generally have an interface to a cyclically clocked communication system. A run level of the automation components (fast-cycle) (e.g. positional control in a controller, torque control of the drive) is synchronized to the communication cycle. This defines the communication timing. Other lower-performance algorithms (slow-cycle) (e.g. temperature controllers) of the automation components can also only communicate via this communication clock with other components (e.g. binary switches for fans, pumps, . . . ), although a slower cycle would be adequate. Using only one communication clock for transmission of all information in the system produces high demands on the bandwidth of the transmission link.
For use in automation systems in particular, PROFIBUS as well as PROFInet are known from the prior art. You can find technical information about these systems at www.profibus.com. In Profibus networks the synchronization of the communication cycles, i.e. what are referred to as the isochronous cycles, are entirely hardware implemented using a Phase Locked Loop (PLL). This is possible since the Profibus topologies have underlying bus structures. Setting up point-to-point connections with such systems is not possible, especially not in an Ethernet-based network. The PLL realized in the hardware of a Profibus system leads to oscillation effects in an Ethernet network.